An example of an electrochemical cell is an electrolyser. In electrolysers, electrical energy is supplied to water to produce hydrogen and oxygen by electrolysis. The electrolyser may contain a solid polymeric electrolyte or a liquid electrolyte.
Electrolysers are employed to produce hydrogen and/or oxygen for various applications, ranging from laboratory gas supplies to refuelling hydrogen-powered vehicles. Electrolysers are usually rated by gas purity and the rate of gas delivery.
A conventional (planar) solid polymer electrolyser consists of a number of cells, each comprising a polymeric membrane (for ion transfer and for separating the oxygen and gas evolution reactions), and two electrodes per cell for providing the electron conduction paths. The electron transfer, ion transfer and gas evolution processes are characterised by “overvoltages” (inefficiencies), and these result in heat generation. Thus heat extraction from the active surfaces of each cell is essential in order to keep the cell temperature below its maximum safe operating temperature.
Conventionally, forced convection cooling of one or both surfaces of each cell is achieved by re-circulating the water used for electrolysis in a pumped thermal circuit employing a heat exchanger for transferring heat to the surroundings. As the water/gas mixture emerges from the electrolyser cells, the gas needs to be separated (usually by means of a separating tower) before water can be returned to the cell(s). (An electrolyser which circulates water on both sides of the membrane requires two thermal circuits with associated pumps, heat exchangers and separating towers). Also water is consumed (due to electrolysis) on the oxygen side, and transmitted by electro-osmosis through the membrane from the oxygen side to the hydrogen side.
These heat generation, water transfer and gas/water separation processes must therefore be managed appropriately during the operation of an electrolyser. This requires a significant set of ‘balance of plant’ (BoP) technologies, which tends to make an electrolyser system complex and expensive.
Good electrical contact is maintained in conventional planar electrolysers by the use of tie rods and stiffened bulky end plates to pressurise the membrane electrode assembly (MEA). This leads to uneven pressure in the MEA and bending stresses. Also, when planar electrolysers are arranged in a stack, it is necessary to maintain sufficient pressure and a good electrical contact between end plates. This leads to further compressive stresses, which can cause failure of the cell.
A significant problem also exists with the servicing of planar electrolysers in a stack. As there are multiple tie-rods and nuts in a stack, a great deal of work has to be done in order to service all of the cells within the electrolyser, and the servicing on one cell can impact on the contacts within all the other cells.